The present invention relates to an improvement of the Leclanche dry cells and more particularly a dry cell which encloses therein an electrolyte consisting mainly of zinc chloride and which has an improved depolarizing mix and a top sealing or closure construction.
The conventional Leclanche dry cells use in general two electrolyte compositions. One is the composition wherein both zinc chloride and ammonium chloride exist in liquid phase (which composition will be referred to as "the composition of the region A" hereinafter in this specification), and the other is the composition mainly consisting of zinc chloride with or without the addition of a minor constituent such as ammonium chloride (which composition will be referred to as "the composition of the region B" hereinafter in this specification). The composition of the region A is generally used in the dry cells from which a large current is intermittently drawn or a small current is continuously or intermittently drawn. But it has defects that the leak-proofness is not satisfactory and that the large current discharge characteristic is also not satisfactory. On the other hand, the dry cells using the composition of the region B exhibit the excellent large and medium current continuous and intermittent discharge characteristics so that they have recently attracted much attention and being further researched and developed for further improvements. For instance, it was reported that when the water content of the depolarizing mix containing the electrolyte mainly consisting of zinc chloride is 23 to 39%, the leak-proofness may be considerably improved. It was further reported that when the content of water relative to that of manganese dioxide, the content of chlorine which is present in the form of zinc chloride and hydrochloric acid, and the percentage by volume of the electrolyte are suitably controlled depending upon the concentration of the electrolyte, the discharge characteristics, the shelf life and the leak-proofness may be considerably improved.
From the standpoint of the depolarizing mix, the conventional dry cells may be divided into the paste electrolyte type and the paper-lined type. Recently, the paper-lined type is more preferred than the paste electrolyte type, and various separators for the paper-lined type have been devised and demonstrated. For instance, the effects of the density of sheets of paper used as the separators as well as the quantity of gelatinous paste applied thereupon have been extensively studied and tested, and there has been proposed a separator in which a film is used to prevent the movement of the gelatinous paste as well as water.
As to the top sealing or closing construction, various constructions consisting of the combinations of a plastic top seal and a metal top have been proposed, but they are still unsatisfactory in practice and do not provide the complete leak-proofness. Furthermore, the problem of the corrosion of a cathode cap by the electrolyte permeated through the carbon electrode during storage cannot be solved merely by improving the top sealing or closure construction.
As to the chemical composition of the depolarizing mix, the inventors made extensive studies and experiments in order to find out the effects of the chemical compositions of the regions A and B upon the discharge characteristics, the shelf life and the leak-proofness. One of the greatest differences between the two is the difference of movement of ions between the anode zinc can and the depolarizing mix during discharge. That is, in case of the composition of the region A, the ions moving during the discharge consist mainly of ammonium ions, but in case of the composition of the region B, the moving ions consist mainly of zinc ions. The discharge characteristics as well as the leak-proofness are much influenced by the difference in moving ions during the discharge, and the chemical composition of the electrolyte as well as its pH considerably affect the gelation of the starch used in the gelatinous paste over the separator and the corrosion of the zinc can. That is, in case of the composition of the region A, electric charge is carried mainly by ammonium ions and the concentration of the electrolyte is high. Consequently, the movement of zinc ions is difficult, and zinc ions dissolved out of the zinc can are trapped in the separator. As a result, there exists an ion concentration difference between the depolarizing mix and the vicinity of the zinc anode, and a thin layer of zinc diaminechloride is formed due to the reaction of ammonium ions diffused over the surface of the depolarizing mix, thus resulting in the rapid concentration polarization. It is this formation of the thin layer of zinc diaminechloride that adversely affects the continuous large current discharge characteristic. However, on intermittent discharge, the zinc ions are diffused during the idling time and the conductivity of the electrolyte is high (of the order of 420 m-moh) so that the satisfactory intermittent discharge characteristic may be attained.
As to the gelatinous paste layer of the separator, it has been proposed to use, as the electrolyte holding or keeping agent, various starches such as corn starch, wheat flour, potato starch and chemically processed starches such as bridged and/or etherificated starches, cellulose ethers and water-soluble synthetic resins. However, the effects of even starches, which are excellent and inexpensive materials, upon the electrolyte have not clarified yet. The hydrogen bonds in the molecular chains of starch are loosened by zinc ions so that the electrolyte may be held between the molecules by hydrogen bonds. In case of the composition of the region A, the zinc ions in the separator are considerably increased especially during the continuous large current discharge so that the starch particles are swelled. As a result, the molecular bonds are gradually weakened and finally collapsed so that the electrolyte is discharged. Furthermore, because of the osmosis due to the difference in concentration, water in the depolarizing mix moves into the separator and leaks from the anode. The ether and hydrogen gas produced when the depolarization capability of manganese dioxide deteriorates, leaks through the gaps in the top seal or closure out of the dry cell. The above is the reason why there are many leaks in the dry cells with the composition of the region A.
In case of the composition of the region B, the major moving ions are zinc ions so that the concentration polarization occurs less frequently. Furthermore, the quantity of the electrolyte is relatively less so that the zinc ion diffusion may be much facilitated. As a result, the continuous large current discharge characteristic is satisfactory; the leakage is less; and the leak-proofness is improved. The discharge characteristics as well as the leak-proofness are much affected by the selection of the swelling degree of starch used.
As to the volume of hydrogen gas produced toward the drop to a final voltage, the experiments conducted by the inventors showed that there is no difference between the compositions of the regions A and B.
Even when manganese dioxide, acetylene black and electrolyte in the depolarizing mix change more or less, the pH of the electrolyte extracted from the region A is 4.8 to 5.4 when 0.8 to 1.5 parts of zinc oxide is mixed to 100 parts of manganese dioxide. On the other hand, the pH of the electrolyte extracted from the region B is 3.8 to 4.5. The zinc corrosion is much affected by the pH value of the electrolyte, and the corrosion proceeds rapidly in the presence of oxygen. Therefore, in case of the composition of the region A, the influence of the air outside of the dry cell must be eliminated.